in brewing beer, the primary use for campden is removing chlorine or chloramine from brewing water. half a tablet is sufficient for the ~6-7 gallons you would use for a 5 gallon batch of beer. crush and dissolve in the water and then let it sit for a while (I am not sure how long, maybe an hour?).
you do this because if the yeast meet chlorine or chloramine they will produce nasty plasticy tasting compounds in your beer. nothing dangerous, just unpleasant.
It is also used in wine/cider/mead making to stun any wild yeast already present on the fruit before adding your known strain. the yeast you add will out compete any remaining populations of weakened yeast or bacteria left in the juice. You have to add the cambden and wait a while (again I am not sure of the time frame here) until it is mostly gone so it doesn't do the same to they yeast you add.
